Evidence linking dysregulation of cell cycle and Bcl-2 family proteins in the molecular pathogenesis of multiple myeloma (MM) has prompted intense interest in cyclin-dependent kinase (CDK) inhibitors and Bcl-2 antagonists in this disease. Furthermore, recent findings suggest that certain CDK inhibitors (e.g., flavopiridol;FP) act as transcriptional repressors by inhibiting the CDK9/cyclinT pTEFB complex, and by extension, phosphorylation ofthe carboxy-terminal domain of RNA Polll. Such agents down-regulate expression of short-lived proteins including Mcl-1, a critical survival factor in MM. Significantly, CDK inhibitors have recently been found to enhance the lethality of Bcl-2 antagonists (e.g. ABT-737) in human leukemia cells by unleashing Bak from Bcl-xL and Mcl-1, leading to a dramatic potentiation of apoptosis. Notably, a novel FP schedule has recently been developed which displays significant activity in another B-cell malignancy (CLL). We therefore hypothesize that clinically relevant CDK inhibitors such as FP, seliciclib (Rroscovitine), and SCH727965, an agent with an 1050 of 1 nM toward CDK9, represent logical candidate agents to enhance the activity of clinically relevant Bcl-2 antagonists (e.g., GX15-070, ABT-737) in MM. Indeed, preliminary evidence suggests a high degree of synergism between FP and GX15070, as well as other CDKI/Bcl-2 antagonist regimens, in MM cells. Evidence also suggests that such regimens induce upregulation of pro-apoptotic proteins (e.g., Bim, NOXA, and BIK) which may cooperate with Mcl-1 downregulation to trigger apoptosis. In specific aim #1, we will employ genetic tools to test the hypothesis that synergistic interactions between CDK inhibitors and Bcl-2 antagonists stem from Mcl-1/XIAP downregulation, upregulation of Bim, NOXA, and BIK, release of Bak and BIM from both Bcl-xL and Mcl-1, Bax/Bak activation, and induction of mitochondrial injury. This information will guide the selection of correlative laboratory studies in subsequent planned clinical trials. In Specific Aim #2, we will determine whether and by what mechanism(s) this strategy overcomes conventional drug resistance, stromal/cell adhesion- or growth factor-mediated drug resistance, and bortezomib or lenalidomide resistance in MM cells. In specific aim #3, we will evaluate the selectivity of this strategy by comparing its activity against primary, patient-derived CDI38* MM versus their normal counterparts (e.g., CDI38", CD34* cells), and testing its in vivo efficacy using flank and systemic xenograft MM models. In Specific Aim #4, we will use this information as a foundation for initiating one or more Phase I trials of CDKIs (e.g., FP) and Bcl-2 antagonists (e.g., GX15-070) in patients with refractory MM. Collectively, these studies will provide a rational foundation for a novel approach to MM therapy in which the activity of clinically relevant Bcl-2 antagonists (e.g., GX15-070 or ABT- 737) is enhanced through rational combination with transcriptionally repressive CDK inhibitors that disrupt the pTEFb complex (e.g., FP, seliciclib, or SCH727965) in patients with refractory MM.